Accelerating set of retarded cement

ABSTRACT

What is disclosed is a method of accelerating the set of a retarded cement. The cement may range from a drilling fluid containing dispersants or retarders that will slow down the set of the drilling fluid when cementitious fluid is added thereto, through overwashing, or treating a retarded primary or remedial cement that may have been injected into fractures or areas within a well to overwashing a plug or any volume of retarded cement that needs to be set in a well. 
     The specific uses are discussed.

CROSS-REFERENCE TO RELATE APPLICATIONS

This is a continuation-in-part application of Ser. No. 07/393,546, filedAug. 14, 1989, now U.S. Pat. No. 4,953,620 same title, same inventorsand same assignee.

FIELD OF THE INVENTION

This invention relates to cementing oil and gas wells. Moreparticularly, this invention relates to accelerating set of cement thatis highly retarded; such as, retarding drilling fluid converted tocement; or retarded conventional cements, including cement slurries thatmight commingle with a drilling fluid or otherwise become contaminatedwith a retarding chemical.

BACKGROUND OF THE INVENTION

The prior art is replete with a wide variety of both inorganic andorganic cement accelerators. In a co-pending application, DocketDP-50-6-980A serial number 07/131,878, now U.S. Pat. No. 4,883,125entitled "CEMENTING OIL AND GAS WELLS USING CONVERTED DRILLING FLUID" ofwhich the present co-inventors are co-inventors and having the sameassignee, there was discussed the completion of oil and gas wells byconverting of a drilling fluid, or "mud", to a cement, and severalpatents were cited therein which disclose compositions for accomplishingthis.

In that application it was noted that efforts to convert drilling fluidcontaining cementitious materials have posed problems such as increasedviscosity due to flocculation as cementitious material is added to thedrilling mud, and subsequently pumped into the wellbore. Therein wascited U.S. Pat. No. 3,499,491, which describes difficulties with gellingcharacteristics and, particularly, temperature sensitivity.

In both primary and remedial applications, it is also sometimesdesirable to set retarded cement formulations more rapidly. Typicallycement is retarded for pumping into wellbore annuli or perforationswhere it is desirable to have a controlled viscosity and set time topermit safe cement slurry placement, but also desirable to have theslurry set rapidly after placement. For example, to allow ample time forplacement of cement formulations before set, retarders are frequentlyadded to prevent the development of high viscosities or prematuresetting of cement slurry during the pumping and squeezing into theannular voids or perforations. Yet, these retarders can also delay thesetting of the cement for long periods of time after placement evenunder in situ conditions. Excessive set times contribute to highoperational costs due to rig time standby. Thus, it is desirable that acompatible accelerator be employed in some fashion to greatly reduce theset time for the cement.

The same is true when setting cement in the form of plugs in thewellbore. This may be for plugging the well; formation pack off, or sealoff between productive formations or otherwise; and/or for wellboredeviation. Frequently, gravity or other factors cause the cement plug tointermingle or become contaminated with the drilling fluid. The drillingfluid is an effective cement set retarder and prevents or delays the setof the cement plug. In any event, it is sometimes desirable that an overwash be provided to accelerate the set of the cement strengthdevelopment without adversely affecting pumping time.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provideimproved composition and method that will increase the rate of set ofretarded cement regardless of what application it is employed in.

It is a specific object of this invention to provide a composition inwhich the drilling fluid can be converted to a cement which will set upwithin a desirable time after the cementitious material is addedthereto.

These and other objects will become apparent from the descriptive matterwhich follows.

In a broad aspect of this invention, there is provided a compatibleaccelerator for accelerating the set strength development rate of aretarded cement in any application.

In accordance with one aspect of this invention, there is provided amethod of accelerating the set of highly retarded cements by overwashinga retarded cement composition with a compatible accelerator that willdecrease the setting time for the cement in annular voids orperforations and/or in a plug in a wellbore.

In accordance with another aspect of this invention, there is provided acement composition for cementing any designated section of a wellborewherein the space in the wellbore is occupied by a drilling fluid. Thecomposition comprises a quantity of the drilling fluid containingviscosity control agents which serve as retarders when the cementitiousmaterial is added; a dry cement material, a dispersant and anaccelerator that is compatible.

By compatible accelerators is meant an accelerator that contains amaterial that will form formate ions without a violent reaction withbasic cement slurry constituents in the cement slurry. We have obtainedexcellent results with formamide and know that it works regardless ofwhether or not the theory regarding formate ion is correct. Othersources of formate ion in cement slurry are salts of formic acid andesters of formic acid. For example, it is known that the first fourcarbon esters of formic acid will react satisfactorily fast to give goodresults. These esters include the methyl ester of formic acid, sometimescalled methyl formate; the ethyl ester of formic acid sometimes calledethyl formate; the propyl, both iso- and normal-propyl ester of formicacid, sometimes called, respectively, iso-propyl formate or normal-propyl formate; and the butyl, iso-, normal-, or tertiary-butyl ester offormic acid, sometimes called iso-, normal- or t-butyl formate. Whetheror not the theory of hydrolysis is correct, we do obtain excellentresults with these first four carbon esters of formic acid and know thatthey work regardless of whether or not the theory is correct.

In another aspect, this invention provides a method for treating a wellin which the subterranean formation about the well may or may not befractured and a retarded cement slurry is applied to seal off annularvoids and perforations. There is also provided the method forimprovement of washing over the cement with a solution of a compatibleorganic accelerator such as the material that will yield formate ions,as delineated hereinbefore.

In another aspect of this invention there is provided a method foraccelerating a cement plug in a well completed in a subterraneanformation where cement is pumped into the well and allowed to set toform a plug therewithin. The improvement comprises either washing overthe cement plug with a solution of a compatible organic accelerator orpumping the plug into an interval of the well previously filled with thecompatible organic accelerator. The compatible organic accelerator isthe material that will yield formate ions in the cement slurry, asdelineated hereinbefore.

These compatible organic accelerators can be employed alone or incombination with other known accelerators, such as but not limited tothose inorganic accelerators like silicates (sodium metasilicate, sodiumsilicate, or potassium silicate) or chlorides (sodium chloride,potassium chloride, or calcium chloride).

DESCRIPTION OF PREFERRED EMBODIMENTS

The conversion of well drilling fluids into cement slurries for thepurposes of cementing in wellbores to casing annulus, and/or performingother wellbore cementing applications is attractive for several reasons;one of which would be that a major portion of the drilling fluid wouldnot be subject to waste disposal regulations Moreover, the conversion ofdrilling fluid to a cement slurry minimizes the handling of drillingfluid after completion of the well, and minimizes the cement expense,and the mud cement will effect a more nearly complete displacement ofthe drilling fluid.

The conversion of drilling fluid, or "mud" to a cement slurry is notwithout some operational problems and undesirable compositional changes.For example, the addition of cementitious material such as mixtures oflime, silica, alumina or lime and magnesia, silica and alumina and ironoxide, or cement materials such as calcium sulfate and Portland cementto aqueous drilling fluids can substantially increase the viscosity ofthe fluid mixture and cause severe flocculation. Efforts to circulatesuch mixtures through a wellbore can result in highly unsatisfactorycirculation rates, restricting flow in the wellbore annulus, increasingpressures, resulting in breakdown of the earth formations in thevicinity of the wellbore. In addition, excessive viscosity contributesto failure of the surface operations to mix the cement slurry properly.Certain dispersants have been developed for use in these drilling fluidsduring the drilling operations. These include lignite, ligno-sulfonates,and polyacrylates. Other dispersants have been used; particularly, inthe situation where it is desired to convert a drilling mud to a setcement. These other dispersants are discussed in co-pending applicationDP 50-6-980A, now serial number 07/131,878.

It is well recognized by those skilled in the art that these normalconstituents of drilling mud also act as retarders, which inhibit thesetting of cement slurries. Specifically, it may take a week or longerfor such converted drilling mud cements to set, and it is desirable toshorten this time.

It is believed helpful to discuss this conversion of drilling mud tocement in greater detail before looking at other embodiments for whichthis invention is useful. The process for converting a drilling mud to acement slurry for cementing a well is discussed in the above-referencedserial number 07/131,878, and does not require detailed explanationherein. Details of that application are included herein by reference.Expressed otherwise, the operation simply comprises a casing that isextended into a portion of the formation from a wellhead with a secondcasing extending into the formation further as a wellbore is deepened toform an annulus which may include washouts or void areas behind thecasing. The casing is adapted to be in communication with the pump forcirculating drilling fluid through the interior of the casing, up theannulus and through a return conduit to a storage tank or pit and isrecirculated through the pump in normal drilling operations.Conventional drilling fluid conditioning devices such as shale shakers,sand separators and related equipment may be employed; but are normallynot shown in schematic illustrations in the interest of clarity. Onemethod for converting a drilling fluid into a cementitious slurry is theaddition of premixed quantities from storage of dry blended cementformulation for conduction to a slurry mixing operation. The formulationis added to a slurry and mixed into the drilling fluid which is therebyconverted into a cementitious slurry. It is at this point that it isconvenient to add accelerators or the like to at least neutralize theset retarding effects of the various dispersants that may have beenpresent in the drilling fluid, or added to the drilling fluid in thecement conversion process.

As described hereinbefore, compatible accelerators comprise thematerials which will yield the formate ions without the violentreaction. We know from experimental data that formamide will provideexcellent results and that the first four carbon esters of formic acidwill provide good results.

These materials are discussed in Hackh's Chemical Dictionary and do notrequire further description herein.

In this invention, a compatible accelerator is employed when aconcentration range of from 0.05 to about 2.5 gallons per barrel oforiginal drilling fluid is present. A better concentration is a range of0.3-1.2 gallons per barrel of cement slurry employed. The concentrationmay vary from this for a specific application. For most applications theoptimum concentration is about 0.6 gallons per barrel of cement slurry.This will result of a setting time for the cementitious slurry ofapproximately twenty-four hours, which is substantially shorter than theprior art retarded cements.

When employed in an over wash, the compatible accelerator is in aconcentration within the range of 5 percent by weight to 100 percent byweight, the remainder, if any, being water.

As indicated, this invention is useful also when an over wash isemployed after squeezing cement through perforations or in casingcracks. A highly retarded cement may be emplaced and then washed overwith a solution containing the compatible accelerator; for example, thefirst four carbon esters of formic acid. The differential pressureextended from the wellbore into the formation will force the liquid thatis employed as the over wash solution containing the cement acceleratorinto the permeable cement matrix. Normally, such an over pressure is inthe range of from 200-500 pounds per square inch (psi) greater in thewell than it is in the formation so it tends to effect flushing of theover wash solution containing the cement accelerator through thehydrating cement matrix.

The compatible accelerator can be employed to mitigate adverse effectsof contamination from in situ mixing with fluids, such as drillingfluids, in any application. Specifically, if too much retarder is putinto a cement slurry, an effective amount of compatible accelerator canbe employed to neutralize the retarder and get it back into the desiredrange.

In another operation, the solution containing the accelerator may bespotted below a cement to be placed in a well to form a plug. Thereaftera spot of liquid containing the cement accelerator may be positionedabove the cement. A surfactant may be employed in a small concentrationof about 0.1-1.0 percent (%), if desired, in the preflush, cementslurry, or overflush, to decrease the interfacial tension and help thesurrounding fluids to penetrate into the cement matrix.

The accelerators have the property of accelerating the set of the cementand have the potential for immediate application in the conversion ofmud to cement and as an over wash additive, or for washing anaccelerating solution from both above and/or below to produceacceleration of cement nodes in remedial squeeze operations, cementplugs in plugback, kickoff, whipstock procedures, or cement at the topof liners after liner cementing operations.

EXAMPLE

The following example illustrates both a control without this inventionand laboratory experiments performed to simulate field conditions andshow effectiveness of the accelerator.

EXAMPLE I

In this example from 350 cubic centimeters (cc's), serving as anexperimental one barrel quantity, of 10.45 pounds (lbs.) per gallon(ppg) Rapid Mud (a trademark of Baroid Corp) was employed with 175 cc'sserving as 0.5 barrel of water, and 300 lbs. per barrel (coarse grind)Kaiser Cement in a quantity of 300 grams was employed with 2.25 lbs. perbarrel (2.25 gram SSMA, which is sulfonated styrene maleic anhydridecopolymer). The result was a retarded cement having a density of 12.3pounds per gallon. This served as a base for comparison and had 0 psicompressive strength and had not even begun to set up after 4 days, butafter 4.6 days it gave a compressive strength of 50 psi and after 11days gave a compressive strength of 500 psi but the pumping time wasstill about 48+ hours at 94 degrees Fahrenheit.

In contrast, 350 cc's of 10.45 ppg Rapid Mud in 175 cc's water, giving300 lbs. per barrel (coarse grind) Kaiser Cement with 3 gms. (lbs. perbarrel) SSMA and 2.4 gal/bbl, or 20 ml., of formamide, gave acompressive strength of 50 psi (pounds per square inch) after only 18hours; 460 psi after 4 days and 562 psi after 11 days. The pumping timeremained unchanged at 48+ hours at 94 degrees Fahrenheit.

Although this invention has been described with a certain degree ofparticularity, it is understood that the present disclosure is made onlyby way of example and that numerous changes in the details ofconstruction and the combination and arrangement of parts may beresorted to without departing from the spirit and the scope of theinvention, reference being had for the latter purpose to the appendedclaims.

What is claimed is:
 1. In a method of setting a volume of cement in awell completed in a subterranean formation in which a cement is or maybecome retarded and is pumped into said well and allowed to set to forma hard cementitious material therewithin, the improvementcomprising:contacting said cement with a solution of a compatibleorganic accelerator comprising a material that will produce formate ionsin the cement slurry and selected from the group consisting of the firstfour carbon esters of formic acid, said esters including methyl formate,ethyl formate, normal-propyl formate, iso-propyl formate, normal-butylformate, iso-butyl formate, and t-butyl formate.
 2. The method of claim1 wherein said compatible organic accelerator is the methyl formatewhich is present in a concentration in the range of 0.5-2.5 gallons perbarrel based on the original drilling fluid present.
 3. The method ofclaim 1 wherein said compatible organic accelerator is the ethylformate, which is present in a concentration in the range of 0.5-2.5gallons per barrel based on the original drilling fluid present.
 4. Themethod of claim 1 wherein said compatible organic accelerator is theiso-propyl formate which is present in a concentration in the range of0.5-2.5 gallons per barrel based on the original drilling fluid present.5. The method of claim 1 wherein said compatible organic accelerator isthe normal-propyl formate which is present in a concentration in therange of 0.5-2.5 gallons per barrel based on the original drilling fluidpresent.
 6. The method of claim 1 wherein said compatible organicaccelerator is the iso-butyl formate which is present in concentrationin the range of 0.5-2.5 gallons per barrel based on the originaldrilling fluid present.
 7. The method of claim 1 wherein said compatibleorganic accelerator is the normal-butyl formate which is present in aconcentration in the range of 0.5-2.5 gallons per barrel based on theoriginal drilling fluid present.
 8. The method of claim 1 wherein saidcompatible organic accelerator is the t-butyl formate which is presentin a concentration in the range of 0.5-2.5 gallons per barrel based onthe original drilling fluid present.
 9. In a method of setting a volumeof cement in a well completed in subterranean formation, in which acement is or may become retarded and is pumped into said well andallowed to set to form hard cementitious material therewithin at linertops and in wellbore plugs, the improvement comprising:contactingdownhole said cement with a solution of a compatible organic acceleratorcomprising a material that will produce formate ions in the cementslurry and selected from the group consisting of formamide, and estersof formic acid, said esters including methyl formate, ethyl formate,normal propyl formate, isopropyl formate, normal butyl formate, isobutyl formate and t-butyl formate.
 10. The method of claim 9 whereinsaid compatible organic accelerator is formamide and said formamide isemployed in a step of overwashing a retarded cement composition with anoverwash fluid; said formamide comprising from 5 to 100 percent byweight of said overwash fluid.
 11. The method of claim 9 wherein saidcompatible organic accelerator is an ester of formic acid and saidformic acid ester is employed in a step of overwashing the retardedcement composition with a compatible accelerator that will decrease thesetting time for the cement, the fluid being called an overwash fluid,said formic acid ester comprising from 5 to 100 percent by weight ofsaid overwash fluid.